A sensor-calibration module for a sensor producing a signal indicative of a characteristic of a user, the sensor including a counter electrode, a reference electrode, and a working electrode in communication with the users blood or interstitial fluids the sensor-calibration module having a first control unit, wherein the first control unit comprises an operational amplifier (op amp) connected in an electrical circuit such that a first voltage V1 equal to the measured difference between the working electrode voltage and the reference electrode voltage provides a first input into the op amp, an applied voltage V2 provides a second input into the op amp, and the output of the op amp is electrically connected to the counter electrode, said sensor-calibration module being configured to calibrate the sensor based on the value of the measured voltage V1 and compensating for an IR drop due to a resistance between the reference and working electrodes. An alternative way of deriving the IR drop is by applying ACC signals at a frequency which is sufficiently high that the inherent capacitance across the Faradaic part of the resistance between the reference and working electrodes appears as a short circuit and obtaining a value for said unwanted resistance by dividing the applied AC voltage by the AC current flowing, and thereafter calculating the IR drop by multiplying the current passing between the reference and working electrodes by the value of said unwanted resistance.
